It does NOT mean that the message has not been sent. It appears randomly whther the message was sent or not. Nobody, including MetroPCS or Coolpad, knows what it means. It is common to Android phones. The most likely explanation that I've seen, is that it shows up when the message isn't sent on the first try, and then when it is successfuly resent, the exclamation point, just doesn't go away.
What amazes me, is that some programmer somewhere, programmed the phone to put up that symbol. but no-one can find that prograqmmer, and ask why it is there.